The present invention relates to iron-base amorphous alloys having improved fatigue and toughness characteristics.
Metals are usually crystalline in their solid state, but selected compositions of metals, when solidified by quenching, lose the initial long-range ordered atomic structure and acquire even in the solid state a structure similar to that of liquids. Such compositions of metals are generally referred to as amorphous alloys. By properly selecting the alloying elements and their amounts, amorphous alloys having better chemical, electromagnetic, physical and mechanical properties than conventional commercial crystalline metals can be obtained. Because of these excellent properties, amorphous alloys have a great potential for use in a wide scope of applications such as electrical and electromagnetic parts, composite materials and fibers. For example, Japanese Patent Application (OPI) Nos. 73920/1976 and 35618/1978 (the symbol OPI as used herein means an unexamined published Japanese patent application) show amorphous alloys having high magnetic permeability characteristics; Japanese Patent Application (OPI) Nos. 101215/1975 and 3312/1976 show amorphous alloys having improved strength and high resistance to corrosion and heat; and U.S. Pat. No. 3,856,513 shows representative amorphous alloys having improved heat stability. Among the amorphous alloys having various distinctive features, iron-base alloys are most promising as materials for making reinforcements in rubber belts and tires, other industrial products such as ropes, because the iron-base alloys can be prepared at low cost, have a higher tensile break strength than existing commercial crystalline metals, involve little or no work hardening and show good balance between strength and toughness. Particularly interesting iron-base amorphous alloys are Fe-Si-B systems which exhibit a high tensile break strength (400 kg/mm.sup.2 or more). These Fe-Si-B system alloys are known to have a much higher heat resistance than any other iron-metalloid base amorphous alloys.
Metallic parts are classified as "static" and "dynamic" parts. For the first type of parts, which are usually subject to static forces, materials that have been proved to have good tensile properties, particularly high tensile break strength, are required. However, with dynamic parts, such as belts, tires, ropes, and machine parts, which rotate, bend, vibrate, or reciprocate at high speed, fatigue characteristics are more important than tensile properties, i.e., tensile break strength properties. These dynamic parts are constantly subjected to cyclic applications of external forces for an extended period and the occurrence of vibrations and other undersired effects in usually unavoidable. The deformation accompanying an actual break down is not as great as what occurs in a tensile test, and the tensile break strength for the actual case is far smaller than the tested value; in an extreme case, a fatigue break may even occur under stresses lower than the yield point. No material having a high tensile breaking strength can be effectively used in dynamic parts unless it has good fatigue characteristics. The mechanical properties of various amorphous alloy systems have been reported in many papers which describe the results of tensile and compression tests. On the other hand, few reports have been made on the more important fatigue characteristics, the exceptions being Masumoto and Ogura et al., Scripta Metallugica, Vol. 9, pp. 109-114, 1975, which report Pd.sub.80 Si.sub.20 amorphous alloy ribbons, and Imura and Doi et al., Japan J. Appl. Phys., Vol. 19, p. 449, 1980 and Japan J. Appl. Phys., Vol. 20, p. 1593, 1981, both of which report Ni-, Fe- and Co- base amorphous alloy ribbons. According to Imura and Doi et al, the fatigue characteristics of Fe.sub.75 Si.sub.10 B.sub.15 amorphous alloy ribbon are comparable to those of the existing crystalline SUS 304 and its fatigue limit (.lambda.e) is 0.0018. In other words, the high tensile break strength of this particular amorphous system is not reflected in good fatigue properties; to the contrary, its fatigue limit is lower than that of the typical commercial alloy.
Japanese Patent Application (OPI) No. 4017/1976 shows an iron-base amorphous alloy having improved resistance to many types of corrosion (i.e., general corrosion, pitting, crevice corrosion, and stress corrosion cracking) and which contains an Fe-(P,C,B)-Cr alloy as the major component and several other elements as auxiliary components. This alloy is described as being suitable for use as reinforcement cords embedded in rubber and plastic products, such as vehicle tires and belts. Particularly, this application is directed to an iron-base amorphous alloy having high strength and improved resistance to fatigue, general corrosion, pitting, crevice corrosion, stress corrosion cracking and hydrogen embrittlement, said alloy containing as the principal components 1 to 40 atom % of Cr and 7 to 35 atom % of at least one element selected from among P, C and B, and as an auxiliary component a total of 0.01 to 75 atom % of an element of at least one of the groups (1) to (4) shown below, with the balance being substantially Fe:
(1) 0.01 to 40 atom % of Ni or Co or both;
(2) 0.01 to 20 atom % of at least one element selected from among Mo, Zr, Ti, Si, Al, Pt, Mn, and Pd;
(3) 0.01 to 10 atom % of at least one element selected from among V, Nb, Ta, W, Ge, and Be; and
(4) 0.01 to 5 atom % of at least one element selected from among Au, Cu, Zn, Cd, Sn, As, Sb, Bi, and S.
The alloy specifically shown in Japanese Patent Application (OPI) No. 4017/1976 is Fe.sub.67 Si.sub.15 B.sub.1 P.sub.13 Cr.sub.3. While this alloy has high resistance to general corrosion, pitting, crevice corrosion, and stress corrosion cracking, the desired amorphous state cannot be obtained from this alloy having low amorphous forming ability and the fatigue characteristics of the resulting amorphous alloy are not as good as expected. In short, this alloy is not completely satisfactory as a material for use in dynamic parts.
An iron-base amorphous metal filament with a circular cross section and a process for producing the same has been described in European Patent Publication (unexamined) No. 39169 (European Patent Application No. 81301624.3 filed Apr. 14, 1981). The amorphous alloy of which the filament is made has high corrosion resistant, toughness, and good electromagnetic properties, and hence is suitable for use in various industrial materials such as electrical and electronic parts, composites, and fibers. Among the alloys specifically shown in this prior application are Fe-Si-B-Cr systems, such as Fe.sub.71 Cr.sub.10 Si.sub.10 B.sub.9, Fe.sub.70 Cr.sub.5 Si.sub.10 B.sub.15, and Fe.sub.50 Co.sub.20 Cr.sub.5 Si.sub.10 B.sub.15. Although Cr is incorporated in these alloys, its presence is intended to provide improved resistance to corrosion and heat, as well as enhanced strength, but not to afford improved fatigue characteristics. Stated more specifically, the alloys with 5 atom % of Cr (Fe.sub.70 Cr.sub.5 Si.sub.10 B.sub.15 and Fe.sub.50 Co.sub.20 Cr.sub.5 Si.sub.10 B.sub.15) have low levels of fatigue characteristics with little improvement achieved by the addition of Cr. The other alloy, with 10 atom % Cr (Fe.sub.71 Cr.sub.10 Si.sub.10 B.sub.9), has low amorphous-forming ability, and the resulting amorphous product does not have a high degree of toughness.
U.S. Pat. No. 4,473,401 describes an iron-base amorphous alloy having improved fatigue characteristics and consisting of not exceeding 25 atom % of Si, 2.5 to 25 atom % of B (Si+B=15 to 35 atom %), 1.5 to 20 atom % of Cr, and the balance being Fe. This alloy had good fatigue characteristics, but on the other hand, it turned out to be somewhat unsatisfactory in toughness. As already mentioned, practical materials which are used in various forms such as twisted, woven, and knitted states should have not only good fatigue characteristics but also high toughness. Materials having improved fatigue characteristics are extremely low in their value as practical products if they do not have great toughness. Practical materials are often put to use after they have been subjected to some deformation, or processed, or treated during the process of making a composite. For example, they are used in a twisted state as reinforcements in rubber belts or tires, or as ropes; in other cases, they are used as filters in a woven or knitted state. Materials that cannot be used after being subjected to such deformation or processing have an extremely limited scope of practical application.
It is generally said that amorphous metals have high toughness. However, this means either that they are tougher than crystalline metals of the same composition (alloy compositions which easily turn amorphous are very brittle in the crysalline state and find no practical uses) or that they are tough for their high degree of strength. In comparison with existing practical materials such as crystalline steel wires and piano wires, the toughness of amorphous metals is rather low. For example, such practical materials can be easily worked by a twisting, weaving, or knitting machine; on the other hand, amorphous wires are subject to frequent breaking when they are worked by the same machine.